The present invention relates to a heat sealable wrapping or packing film which is capable of forming a peelable seal. The seal is achievable either between two films of this kind, or between one film of this kind and a polypropylene packing film without the need for an adhesive between this film and the polypropylene.
A peelable seal is defined to be the seal or joint between two films produced by heat sealing or impulse sealing, the joint thus formed having the property of being able to open in the original plane of joining of the two films by the action of a pulling force, without wrenching off or tearing occurring in the material of the two films used to make up the joint. For the purposes of the present invention, the peelable seal must possess a mechanical resistance sufficient to maintain the wholeness and the tight-seal properties of the packing and wrapping during storage and transport until such time as the packing or wrapping is opened by the user of the article. The mechanical resistance of the peelable seal must be low enough to permit ready manual opening of the joint, i.e., without the use of any auxiliary instrument.
The invention relates to blends, and a method for making a blend usable in a flexible film package. This invention relates to films and/or laminates which are characterized by a nearly constant peel strength over an extended heat seal temperature range and by being peelable. The term "peelable" refers to a film having seal failure which occurs primarily at the interface of the sealing surface, and not by film tearing. The blends, films and/or laminates of the present invention permit the manufacture of a more consistent finished product, having a seal of predictable and constant peel strength, in spite of inevitable variations in the heat seal temperatures used in the production of such packages.
In the past, many variations of thermoplastic materials have been employed in the manufacture of films capable of forming peelable seals. See, for example, U.S. Pat. No. 4,189,519, to American Can, which discloses a blend for producing a peelable heat seal comprising (1) about 50 to 90 percent by weight of a copolymer of about 80 to 96 percent by weight ethylene and about 4 to 20 percent by weight of an ethylenically unsaturated ester, and (2) about 10 to 50 percent by weight of a crystalline isotactic polybutylene. While capable of forming a peel seal, the film of '519 discloses polybutylene as a minor component. The blend of '519 bonds to high density polyethylene (HDPE) without the use of adhesive, but will not bond to polypropylene without an adhesive.
U.S. Pat. No. 3,900,534 to Mobil Oil Corporation discloses thermoplastic shrink films with good heat seal characteristics and good optical properties, however, '534 does not address the need for a peel seal film.
U.S. Pat. No. 3,879,492 to UCB S.A. Belgium discloses blends of polybutylene+styrene-butadiene copolymer+LDPE+HDPE+polyisobutylene.
U.S. Patent No. 4,539,263 to E.I. DuPont de Nemours & Co. discloses peel seals based on blends of comonomers and propylene/.alpha.-olefin copolymer, however, '263 does not provide disclosure directed to polybutylenes.
U.S. patent application Ser. No. 750,342 discloses blends of polybutylene +EVA (or polyethylene) +polypropylene with polypropylene being less than 15 weight percent in the blend. However, non of these references teaches the novel invention.
A variety of other references teach heat sealable films capable of forming peels seals, such as U.S. Pat. Nos. 4,550,141 4,539,263, and 4,414,053. However, none of these references appear to teach the components of the film for forming an intimate packaging film structure as disclosed herein. For example in one embodiment, the instant invention teaches a method of blending a separate tie layer adhesive into a film such that the equipment needed to dispose the tie layer adhesive on the substrate is no longer necessary. The instant invention recognizes that conventional multilayer peelable seal films or sheets are comprised of substrates and peelable sealants which are generally not chemically compatible and accordingly, the present invention teaches a novel blend, and method of making a film which creates chemically compatible substrates and peelable sealants. The instant invention avoids delamination layers when sealed layers are pulled apart. In some embodiments a tie adhesive is utilized to bind incompatible substrates with sealants without the need for additional equipment to bind the adhesive between the sealant and the substrate. The novel and inventive method reduces manufacturing costs for producing peelable seals.
There has been a long felt need for a wrapping or packing material having easy peelability at the seal and yet which will bond to a substrate without the aid of an adhesive.